Prehistoric rock art at many sites, from deep French caves to North American canyons, typically appears in inaccessible locations. Steven Waller, a biochemist and rock-art acoustician in California, thinks he knows why.
After analyzing more than 100 sites on three continents, he found that the places where the art was made tend to have particularly strong sound reflections. "Many legends explain echoes as supernatural spirits, so maybe the artists were going into caves seeking the spirits," Waller says.
Ancient artists may also have chosen the subjects of their paintings based on images suggested by acoustic ricochets. Echoes of hand claps sound like hoofbeats, Waller says, and hoofed animals show up frequently in rock art.
"It makes a kind of audiovisual effect: If you yell at the painted image, it will speak back to you," he says.
Waller hopes his discovery will influence preservation efforts, which often ignore the acoustic impact of changes made to protect an area or to improve access. "The city of Albuquerque is proposing a six-lane highway right through the heart of Petroglyph National Monument in New Mexico," Waller says.